feat(ocr): add guided OCR mode using existing annotation regions

When a document has manually drawn annotation boxes, the user can now
enable "Nur annotierte Bereiche" in the OCR trigger panel. The engine
skips layout detection entirely and runs recognition only within the
pre-drawn bounding boxes, preserving manual transcription blocks.

- Python: adds OcrRegion model, extend OcrRequest/OcrBlock; guided
  branch in /ocr/stream groups by page and crops each region
- Engines: add extract_region_text() to both Kraken and Surya
- Java: adds OcrBlockResult.annotationId, OcrClient.OcrRegion,
  TriggerOcrDTO.useExistingAnnotations; OcrAsyncRunner dispatches to
  upsertGuidedBlock when annotationId is present; OcrService threads
  the flag through to runSingleDocument
- TranscriptionService: adds upsertGuidedBlock (creates, updates OCR,
  or preserves MANUAL blocks)
- Frontend: guided OCR toggle in OcrTrigger shown when blocks exist;
  skips destructive-replace confirmation in guided mode

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/routes/documents/[id]/+page.svelte

@@ -144,14 +144,14 @@ let ocrErrorMessage = $state('');
 let ocrPollTimer = $state<ReturnType<typeof setInterval> | null>(null);
 let ocrSkippedPages = $state(0);
 
-async function triggerOcr(scriptType: string) {
+async function triggerOcr(scriptType: string, useExistingAnnotations: boolean = false) {
 	ocrRunning = true;
 	ocrErrorMessage = '';
 	try {
 		const res = await fetch(`/api/documents/${doc.id}/ocr`, {
 			method: 'POST',
 			headers: { 'Content-Type': 'application/json' },
-			body: JSON.stringify({ scriptType })
+			body: JSON.stringify({ scriptType, useExistingAnnotations })
 		});
 		if (res.ok) {
 			const data = await res.json();